The National Cooperative Highway Research Programs Report, NCHRP Report 350, specifies criteria for evaluating the safety performance of various highway devices, such as crash attenuators. Included in NCHRP Report 350 are recommendations for run-down deceleration rates for vehicles to be used in designing crash attenuators that meet NCHRP Report 350's test levels 2, 3 and 4.
To meet the criteria specified in NCHRP Report 350, most crash attenuators that are deployed today along roadways to redirect or stop vehicles that have left the roadway use various structural arrangements in which the barrier compresses and/or collapses in response to the vehicle impacting the barrier. Some of these crash attenuators also include supplemental braking systems that produce a constant retarding force to slow down crashing vehicles, despite variations in the mass and/or velocity of the vehicle impacting the barrier.
The guidelines in NCHRP Report 350 for crash testing require a maximum vehicle occupant impact speed which is the speed of the occupant striking the interior surface of the vehicle, of 12 meters/second, with a preferred speed of 9 meters/second. Typically, constant braking force crash attenuators will stop a smaller mass vehicle in a distance of around 8 feet. This is because most constant braking force crash attenuators need to exert an increased braking force that will allow larger mass vehicles, such as pickup trucks, to be stopped in a distance of around 17 feet.